Measurement systems and methods as described above exist and they are in particularly used for remote measurement of ocular accommodation of a distant (human) eye.
In clinical practice, the objective measurement of ocular accommodation is necessary for patients who are unable to undergo a subjective refraction test that requires a judgment and response from the individual (such as very young infants). The aim is then to measure the refractive condition (typically near- or farsightedness) of the eye to determine the strength of the prescription glasses. In addition to clinical applications, also in human vision research, continuous measurement of accommodation is used to obtain insight in the physiology and the dynamic behaviour of the eye.
The oldest way to measure the refractive condition in an objective way was by direct observation of the projection of a moving light source on the retina, known as retinoscopy. The retinal projection of the light source causes a retroreflection on the retina of which the movement is indicative for the refractive condition.
As early as 1619, a method was introduced by Scheiner, replacing the moving light by an illumination through a punctured plate, which is placed close to the eye. The holes in the plate essentially create luminous pattern comprising a discrete set of light rays which are to converge on a single point on the retina, i.e. the sensor surface of the eye, in case of proper accommodation. The appearance of multiple retinal projections is again an indication for near- or farsightedness. The Scheiner principle still forms the basis for modern automatic refractometers, or autorefractors.
Warshawsky uses the Scheiner principle to build a mechanical autorefractor as described in “High-resolution optometer for the continuous measurement of accommodation”, Journal of the Optical Society of America, vol 54, nr. 3 pp 375-379, March 1964. Similar methods and systems are described in Campbell et al in “High-speed infrared optometer” Journal of the Optical Society of America, vol 49, nr. 3, March 1959 and Okayama et al in “Eye-tracking infrared optometer”, Ophthalmic and Physiological Optics, Vol. 10, July 1990.
Most of the modern autorefractors continue to be based on the Scheiner principle. All known devices, however, have the disadvantage that the systems and methods are obtrusive. Many systems and methods require the user to look straight into the measuring device, which is quite obtrusive and does not mimic a natural behaviour. For instance, although the autorefractor by Okuyama provides a view on natural targets, the obstruction by the semitransparent mirrors in the vicinity of the eyes and the need for a chin-rest do not create a natural viewing experience.
It is well known that many people when put in an unnatural position and clearly put under observation feel uncomfortable and this may affect such phenomena as heart rate and blood pressure which may have an effect on vision. Apart from the obvious disadvantage of possibly placing the observed object under stress, there is also the disadvantage of obtaining results which do not actually reflect the eye accommodation in normal situations, but the eye accommodation when the subject is put under stress.